Do my self-launched art prints have to be available in all sizes?

No, your self-launched art prints do not need to be available in all sizes. You should upload the highest quality file that you have so that it can be produced in as many sizes as possible for your customers. But even if your file is only large enough to print at 5x7, that's ok! When processing your file, the self-launch tool will automatically select all the sizes that will work for your file.